"Even suggestions of temporary geoengineering to relieve the most dangerous aspects of global warming will be met with so many of the above issues that there will never be world agreement to implement. I can imagine worse scenarios, including global nuclear war started in response to unilateral geoengineering implementation."
But is it possible that a nation could avoid both the insurmountable obstacle of obtaining world agreement, and the possibility of severe sanctions over unilateral action, by doing it in a manner that could not be detected?
Given that the actual results of geoengineering would most likely be lost in the noise, is it actually technically possible to perform SRM geoengineering on a large scale in a manner that could not be detected? Or will all the possible schemes be impossible to hide, being either too visible in deployment, or leaving some tell-tale physical or chemical signature?
One could approach the question from two directions, firstly from the point of view of the rogue nation; if you were to devise a clandestine yet effective SRM scheme, then what would it consist of? Modifications to the nation's domestic jet fuel? A secret fleet of spray tankers operating from a remote base? Ballistic big guns in the wilderness? Atomic bombs in volcanoes?
Secondly, from the point of view of the world community, if you suspect such a thing might happen (or even already be happening) then how do you go about detecting it? Unaccounted radiative forcing changes? Changes in atmospheric sulphur loading? If we get a moratorium on SRM, then how do we "trust, but verify".
I think the question is important from a governance and global politics point of view, but it's also interesting as a surprisingly large and vocal number of people actually believe that covert geoengineering is currently being carried out by governments. See: Public understanding of solar radiation management, A M Mercer, D W Keith, and J D Sharp:
3.4. Conspiracy theories
One of the voices emerging in the SRM debate is that of ‘chemtrails’ believers. This small group believes that organizations, such as governments, are already distributing chemicals in the atmosphere for a variety of purposes, ranging from culling the population to mind control. We found that 2.6% of the subjects believe that it is completely true that the government has a secret program that uses airplanes to put harmful chemicals into the air, and 14% of the sample believes that this is partly true.
The most common purpose ascribed to the supposed spraying is actually covert geoengineering. A better understanding of what covert geoengineering would entail, and how it might be detected, would allow a more rigorous debunking of the various conspiracy theories.